A comic book supervillain, owned and published by DC Comics. He is former Gotham City district attorney Harvey Dent -- and a member of Batman's rogues gallery.
After a mobster scarred one side of his face with acid in the middle of a criminal trial, Dent became obsessed with duality and the number "2". He began flipping a two-headed coin (with one side defaced) to make moral decisions. If the "good" side came up, he'd do the right thing; if the "bad" side came up, he'd do something evil.
Two-Face is almost literally split into two different people. One half of his face is young and handsome, while the other half is hideously scarred. His clothing follows the same pattern: one half of his suit is fashionable and expensive; the other half is seedy and threadbare.
I think Two-Face has always been my favorite comic book villain. His psychosis makes a strange kind of sense, and he's a brilliantly realized character. I like his sense of style, too. He's the reason I'm constantly on the lookout for two-headed coins and weird bisected suits.
The characters has been portrayed in multiple other media. In movies, Billy Dee Williams played Harvey Dent in the 1989 "Batman" film; he'd been promised that he would play Two-Face in any sequels, but Tommy Lee Jones was given the part in "Batman Forever" in 1995. Aaron Eckhart played Dent and Two-Face in "The Dark Knight" in 2008.
On TV, Nicholas D'Agosto played Dent on "Gotham." The character never appeared in the 1966 "Batman" series, but scripts were developed featuring the character -- none other than Clint Eastwood was supposedly tapped for the role at one point.
In animation, Richard Moll voiced Dent and Two-Face in the DC Animated Universe, including "Batman: The Animated Series" and "The New Batman Adventures." In "The Lego Batman Movie," Billy Dee Williams finally gets to play Two-Face, and in "Batman vs. Two-Face," an animated film based on the 1966 TV series, Dent and Two-Face are voiced by William Shatner.